Chromosomes, Genes and DNA
In this topic you will:
- learn that chromosomes contain genes
- find out what genes and chromosomes are made of
Key Words
- chromosomes
- DNA
- genes
Chromosomes
In Stage 7, you learnt that the nucleus of a cell controls its activities. In this unit, you will learn much more about how it does this.
The nucleus of every cell contains threads called chromosomes. Chromosomes were discovered in the nineteenth century. This was when scientists first had sufficiently powerful microscopes, and were first able to see very small things, such as the structure of cells.
When you looked at cells using a microscope, you almost certainly did not see any chromosomes. This is because chromosomes only become visible with a light microscope when a cell is dividing. Chromosomes also need to be coloured using special stains, in order to be able to see them.
This photograph shows a plant cell that is just beginning to divide into two cells. The cells have been stained with a special dye that colours the chromosomes dark red. You cannot really see any of the other structures in the cell.
You cannot see the cell’s other parts clearly in this image, but the chromosome structures stand out due to the stain.
Quick Fact
Human cells usually contain 46 chromosomes, arranged in 23 pairs. These carry all the genetic instructions for the organism.
Important Concept
Chromosomes are thread-like structures found in the nucleus of every cell. They are only visible under a light microscope when the cell is dividing and have to be stained to be seen clearly. Chromosomes carry genetic information that controls the activities of the cell.
Chromosome Numbers in Different Organisms
This photograph shows a chromosome from a fruit fly. Fruit flies have especially thick chromosomes, called giant chromosomes, which are much easier to see than most chromosomes. You can see that this chromosome is a long, striped thread.
Different species of organisms have different numbers of chromosomes in the nuclei of their cells. Human cells have 46 chromosomes. Fruit fly cells have eight chromosomes. Mango trees have 40 chromosomes.
Scientists number the chromosomes in a cell according to how long the chromosomes are. In a human cell, the longest chromosome is chromosome 1, the next longest is chromosome 2, and so on.
We have two of each kind of chromosome. The photograph of the human chromosomes above was made by cutting and pasting pictures of the individual chromosomes. They are arranged in size order, although they do not really line up this way in a real cell.
If you look closely, you may see that each chromosome looks like a cross shape. That’s because each chromosome has been copied before the cell divides, and the two identical copies remain joined together part-way along their length.
Quick Fact
Humans have 23 pairs of chromosomes—one set from each parent—making a total of 46 chromosomes in most body cells.
Common Mistake
Students often think chromosomes always look like an “X”. In reality, they only have that shape when they’ve been copied before cell division. At other times, they are single threads.
Questions
Show Answer
Chromosomes are usually too thin to see. They only become visible when the cell is dividing and have been stained with special dyes.
Show Answer
No, red blood cells do not contain chromosomes because they have no nucleus. Chromosomes are found in the nucleus of cells.
Genes
Each chromosome is made up of hundreds of different genes. The genes are arranged in a particular sequence along the chromosome. Each gene helps to control a particular characteristic in the organism. On this diagram of a short chromosome, the stripes represent different genes.
Scientists are still finding out which genes are found on each type of chromosome in humans. On chromosome 1, for example, we know that there are about 2000 different genes. Chromosome 15 is a much shorter chromosome, and it has about 600 different genes.
Discovering exactly what each of these genes does is not easy, but scientists are learning more all the time. For example, we know that two genes on chromosome 15 help to determine eye colour. Everyone has genes for eye colour in the same place on their chromosome 15s.
But there are different versions of these genes, so one person could have a chromosome 15 with eye colour genes that give them blue eyes, and another could have a chromosome 15 with eye colour genes that give them brown eyes. That is why the two sisters in this photograph have different eye colours.
Differences in gene versions are called alleles, and they help explain why even siblings can look different from one another.
Important Concept
Genes are short sections of DNA found on chromosomes. They control specific characteristics, such as eye colour. Each gene is located at a specific position on a chromosome, and different versions of the same gene (called alleles) can lead to variation between individuals.
DNA
Chromosomes are made of a chemical substance called DNA. Each chromosome is one enormously long molecule of DNA.
This means that genes are also made of DNA.
A DNA molecule has a shape like a twisted ladder. This shape is called a double helix. One gene could be a length of DNA with about 2500 of these twists. We cannot see these twists when we use microscopes to look at chromosomes because DNA molecules are much too small to see.
DNA was first discovered in the 1950s. Since then, scientists have found out a great deal about how the DNA in genes helps to determine the characteristics of humans and other organisms. The DNA in a cell determines what the cell does. It contains a complete set of instructions to make a functioning cell, and a whole organism.
If you continue to study science to IGCSE or O level, you will find out much more about this.
Common Mistake
It's easy to think that you can see DNA when looking at chromosomes under a microscope. But you’re only seeing large structures made of DNA — the double helix itself is far too small to be seen without special technology.
Questions
Show Answer
A chromosome is a long strand of DNA that contains many genes. A gene is a specific section of DNA on a chromosome that controls a particular characteristic.
Show Answer
The difference is caused by different versions of the gene, called alleles. One version codes for normal wings, while another codes for tiny wings.